U.S. Secretary of State Hillary Clinton says the Syrian government has a better chance to avoid what she called a "catastrophic assault" the sooner the violence ends and the political transition begins.

Clinton said the regime of Bashar al-Assad must acknowledge that its days are numbered, and that there was no doubt Syria's opposition forces were growing more effective.

She said last month was the deadliest for the Syrian people in the 16-month revolt against Assad.

Clinton was speaking in Tokyo on the sidelines of an international donors conference for Afghanistan.

Special UN envoy Kofi Annan, meanwhile, acknowledged in an interview published on July 7 that the international community's efforts to find a political solution to the escalating violence in Syria have failed.